Cathode ray tube containing silicon sensitized rare earth oxysulfide phosphors

ABSTRACT

Rare earth oxysulfide phosphors with excellent optical properties and optimum particle size control for use as cathodo-luminescent phosphors in cathode ray tubes are prepared by treating a solution of at least one salt of a rare earth metal and a salt of a rare earth activator to form solid salts and thereafter heating the oxidic compound with a sulfidizing agent and a silicon sensitizer, preferably in the presence of a fluoride. The rare earth oxysulfide phosphors thus obtained contain from 10 to 1,000 p.p.m. silicon sensitizer and, if used, 20 to 500 p.p.m. fluoride, incorporated in the crystal lattice structure.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 392,108, filed Aug. 27, 1973,now U.S. Pat. No. 3,904,546, which application is a continuation-in-partof application Ser. No. 185,891, filed October 1, 1971 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to rare earth phosphors. More particularly, theinvention relates to a method for preparing such phosphors (generally inassociation with a rare earth activator) which involves a treatment witha silicon-containing sensitizer material, and to the resultant rareearth phosphors (and associated activator) incorporating retainedsensitizer and, optimally, fluorine. The term "rare earths" as used inthe present specification refers to yttrium and scandium plus the metalsin Group III of the Periodic Table generally classified as lanthaniderare earths, to wit: lanthanum, cerium, praseodymium, neodymium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,thulium, ytterbium and lutetium. The term "phosphor" refers to amaterial which is capable of exhibiting luminescence when subjected toappropriate excitation. The term "rare earth activator" refers tocompounds of rare earth elements which may be combined with other rareearth compounds to activate luminescence thereof, including, forexample, compounds of europium, terbium, erbium, thulium, dysprosium,ytterbium and proseodymium.

2. Description of the Prior Art

Luminescent properties of certain rare earth-containing compositionshave long been recognized. In recent years, rare earth phosphors havereceived considerable attention and have been the subject of manyintensive investigations. The recent interest is due partly to thediscovery that certain rare earth phosphors, particularly theoxygen-bearing phosphors (oxyphosphors) can be used advantageously ascathodoluminescent coatings for color television tubes. In general, therare earth phosphors are in the form of a solid solution having a matrixof rare earth compounds such as a rare earth oxide or vanadate and anactivator which is commonly called a "dopant" and generally is also arare earth element.

The effectiveness of the activator is dependent to a large extent on itsintimate relation within the rare earth matrix. To insure the formationof an intimate mixture, rare earth phosphor manufactures may prefer todissolve into an acid solution the rare earth element in the form of anoxide, together with the activator, to form a homogeneous solution. Therare earth element and the activator are then coprecipitated fromsolution in the form of oxalate, hydroxide, carbonate or sulfate. Theprecipitate may be recovered and fired at a high temperature todecompose the salts into mixed oxides in powder form. This finelydivided, reactive form is favorable for reaction with certain oxy-acidsto synthesize such oxygen-dominated and europium-activated phosphors asyttrium vanadate, gadolinium vanadate, yttrium tungstate, yttriumgermanate, gadolinium aluminate, etc., and conditions can be adjusted toyield desirable crystal growth and particle size distribution. Thephosphors thus synthesized may be used as luminescent coatings for colortelevision tubes and other applications.

Prior art methods reported for preparing various rare earth activatedoxysulfides and prior art disclosures of various rare earth activatedoxysulfide phosphors are to be found in U.S. Pat. Nos. 2,462,547;3,418,247; 3,418,246; 3,423,621; 3,502,590; 3,515,675; 3,562,174; and3,563,909. Many of these methods involve the reaction of a rare earthcompounds with a sulfur-containing gas at an elevated temperature toform the oxysulfide. Still another technique reported, only for thepreparation of europium activated lanthanum, gadolinium, yttrium andlutetium oxysulfides, is the solid state reaction between the mixed rareearth oxides (activator and host) and a composition such as alkali metalcarbonates and sulfur which produces alkali metal sulfides andpolysulfides, which in turn react at elevated temperatures with the rareearth oxides to form the rare earth oxysulfides. In addition to thealkali metal carbonate, it is reported that an alkali metal sulfate,phosphate, arsenate, or germanate may be used.

U.S. Pat. No. 3,415,757 discloses fluoro-substituted europium activatedgadolinium, yttrium and lanthanum oxide phosphors (oxyfluorides) havinga fluorine content lying between 0.1 and 5.0% by weight. These productsare made by heating a mixture of rare earth oxide with ammoniumfluoride, or preferably europium fluoride.

Although the phosphors described above are being employed as luminescentcoatings for color television tubes and in other applications, thesephosphors do not generally have the combined properties of being freeflowing, having high brightness and controlled particle size. Themethods described above do not suffice for controlling the crystalgrowth properly, and generally lead to wide particle size distributionor result in crystal growth at the expense of lowering the phosphorsoptical properties.

SUMMARY OF THE INVENTION

Through the use of a silicon sensitizer, preferably together with afluoride, and using solid state techniques, we have been able to developa synthetic procedure for producing oxysulfide phosphors in which asmall quantity of the sensitizer, and fluorine, if used, areincorporated in the oxysulfide lattice and which form well defined,non-aggregated phosphor crystals. The phosphors produced arefree-flowing and show unusually high brightness when excited byultraviolet radiation, cathode rays, and X-rays. The new phosphors areeminently suited for use in aircraft cockpit cathode ray displays andother such displays that must be viewed under intense ambient lightconditions. By controlling the amount of silicon sensitizer and fluorideand the temperature of preparation, we have grown sensitizer-containingand sensitizer- and fluoride-containing, rare earth oxysulfide crystalslarger than 20 microns in size.

Broadly stated, the process of the present invention comprises treatinga homogeneous solution of at least one salt of a rare earth host metalselected from lanthanum, gadolinium and yttrium, and a salt of at leastone rare earth activator selected from terbium, europium, praseodymium,erbium, ytterbium, neodymium, thulium and dysprosium, to decompose thesalts to form therefrom an oxidic compound of the rare earth metal andactivator. A mixture containing the oxidic compound, a sulfidizingagent, and a silicon sensitizer, with or without a fluoride, is thenheated to a temperature in the range from 650°C. to 1350°C. to formcrystalline rare earth oxysulfide phosphor containing about 10 to 1,000p.p.m. silicon, and 20 to 500 p.p.m. of fluorine, if used, incorporatedin the crystal lattice.

Preferably, the rare earth oxidic compound is prepared by the processwhich comprises the steps of preparing a homogeneous solution of atleast one rare earth host metal salt and at least one activator salt.The rare earth host metal and the activator preferably are thencoprecipitated. After the precipitate is recovered, it is fired to causeformation of the oxidic rare earth compound.

The silicon sensitizer, and the fluorine if used, can also be introducedinto the oxysulfide by treating a homogeneous solution of at least onesalt of a rare earth host metal, at least one salt of a rare earthactivator, and a silicon compound, and optionally a fluoride salt, todecompose the salts to form therefrom an oxidic silicon and, if used,fluoride-containing compound of said rare earths. A mixture containingsaid compound with a sulfidizing agent is then heated at a temperaturefrom about 650°C. to 1350°C. to form a rare earth oxysulfide phosphor incrystalline form containing about 10 to 1,000 p.p.m. silicon and whenused, 20 to 500 p.p.m. fluorine, incorporated in the crystal lattice.

The rare earth oxysulfide phosphors of the present invention consistessentially of a rare earth oxysulfide matrix of the formula R₂ O₂ Swherein R is a rare earth host element of the group consisting oflanthanum, gadolinium and yttrium, with a rare earth activator of thegroup consisting of praseodymium, neodymium, samarium, europium,terbium, dysprosium, holmium, erbium, ytterbium and thulium in an amountfrom 0.1% to 10% by weight of the phosphor incorporated in said matrix,and with the small amount previously noted of silicon and, optionally,fluorine, incorporated in said matrix. These phosphors are useful asphotoluminescent and cathodoluminescent phosphors for color televisionand specialty cathode ray tube applications. Particularly advantageousphosphors are those in which the host matrix is yttrium or gadoliniumoxysulfide and the rare earth activator is europium or terbium. In thecase of terbium-activated phosphors, the presence in combination withthe terbium of a small concentration of dysprosium, say 10 to 100 partsper million, is often advantageous.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To carry out the process of this invention, the initial steps of formingan intimate mixture containing the rare earth metal values for thephosphor matrix and the activator in predetermined proportions may beaccomplished by conventional techniques well known to one skilled in theart. Thus, one may obtain a homogeneous solution by dissolving thereinthe soluble compounds of rare earth elements as well as the activator.In general, the sulfates, nitrates and chlorides are the rare earthcompounds most commonly used to prepare the homogeneous solution.

The amount of rare earth matrix salt and activator salt that isdissolved in the solvent is not critical. The maximum amount of rareearth that can be used is generally governed by the solubility of thecompounds in the solvent. When a mineral acid is used as a solvent, theconcentration of the acid dictates the concentration of the solutestherein.

Advantageously the proportion of the matrix rare earth and the activatorin the solution is in substantially the same stoichiometric ratio as inthe final phosphor product. The amount of activator in the phosphorvaries within a wide range depending on a number of factors. If desired,two or more activator rare earths may be used in combination with thematrix rare earth, as is known in the art.

The rare earth element and the activator are coprecipitated from thehomogeneous solution by adding thereto a precipitating agent well knownin the art. Thus, the coprecipitation may be accomplished by adding tothe solution oxalic acid, tartaric acid, or ammonium carbonate toprecipitate therefrom oxalates, tartrates and carbonates, respectively.

In the process of this invention, we prefer tp use oxalic acid tocoprecipitate the desired rare earth metal values from the solution.This may be performed, for example, by adding an oxalate or oxalic acidsolution to the homogeneous solution containing the matrix rare earthvalues and the activator. The mixing of oxalic acid with the solutionmay be carried out over a wide temperature range, e.g., 10°C to 70°C.The pH of the resultant aqueous mixture, however, preferably isadjusted, for example, to a value of about 2 to 3, so that it is belowthe value at which the rare earth hydroxides precipitate.

The initial concentration of the solution, the amount of the oxalic acidadded thereto, the pH of the final solution and the temperature for thecoprecipitation all have influences on the type of crystals obtained andthe size of the crystals. In general, it is advantageous to adjust thesevariables so as to effect a coprecipitation that will produce phosphorswith optimal properties.

The rare earth coprecipitate thus obtained is recovered by digesting andfiltration followed by washing and drying. The coprecipitate is thenfired at a temperature sufficiently high to cause formation of therespective rare earth oxides. Generally, firing the recoveredcoprecipitates at a temperature of 550°C. to 1350°C. is eminentlysuitable for the conversion of the salts to rare earth oxides. Theproduct thus obtained is now ready for treatment to convert the oxide tothe oxysulfide phosphors of the invention.

The sensitizer to be used is silicon or a silicon compound. The siliconsensitizer can, for example be added to the reaction in the form ofelemental silicon, or as SiO₂ in any desired form such as colloidedsilica, or in the form of other silicon compound (e.g. an alkali metalsilicate silicon sulfide, or any other siliceous material which does notintroduce undesirable elements into the phosphor product) which willreact to include silicon in the phosphor lattice structure. If fluorineis also to be included, then both silicon and fluorine can be added tothe reaction in the form of a silicofluoride, such as Na₂ SiF₆ orfluosilicic acid.

The treatment is performed by heating together a well mixed mixturecontaining the above-mentioned rare earth oxides and silicon sensitizer,a sulfidizing agent, and optionally; a compound containing fluorine, toform a rare earth oxysulfide phosphor containing 10 to 1000 p.p.m. ofsilicon and, when used, 20 to 500 p.p.m. of fluorine. The heating of themixture may be carried out within a range of temperatures which we havefound may be from 650°C. to 1350°C.

The particular fluoride compound used to incorporate the fluorine withinthe lattice structure of the phosphor can be selected from numerousfluorine-containing compounds. Preferably it comprises a fluoride of analkali metal or ammonium, and it may with advantages be included in amixture containing potassium phosphate. For example, it may be providedas K₃ PO₄ + NaF + NH₄ Cl, or NH₄ F, or NH₄ F.HF, or LiF, or NaF, orKF.2H₂ O, or NaF + KF.2H₂ O, or a fluoride of the rare earth host.However, the particular fluoride compound used is by no means limited tothese particular chemical compounds; it may comprise any other fluoridecompound or mixtures thereof known in the art.

In order to form the oxysulfide phosphor from the oxide phosphor, it isnecessary to heat the oxide phosphor with a sulfidizing agent. So far asthis invention is concerned, any of the sulfur bearing materials orcombination of sulfur-bearing materials which are known in the art forconverting rare earth oxides to rare earth oxysulfides may be used. Forexample, elemental sulfur, alkali metal sulfides and polysulfides, rareearth sulfides, hydrogen sulfide, organic sulfides, alkali thiocyanates,alkali sulfates plus carbon, etc., may be used. Elemental sulfur ispreferred.

We have also found that silicon and fluorine can be introduced intocombination with the rare earth components prior to the sulfidizingtreatment. Therefore, a second embodiment of the method of the presentinvention comprises the initial steps of forming an intimate mixture ofa homogeneous solution containing the matrix rare earth metal salt andthe rare earth activator salt in pre-determined proportions as describedabove. To the solution containing rare earth values is added a siliconcompound, and optionally a fluoride-containing solution in which thereis dissolved, for example, NH₄ F.HF, HF, NaF, or other soluble fluoride.When a fluoride is used and the solution is prepared or held in a glasscontainer, sufficient silicon (upwards to 10 p.p.m. but generally lessthan 1000 p.p.m.) is incorporated in the solution by leaching from theglass walls of the container to make it unnecessary to add a siliconcompound from any other source.

The resulting solution of rare earths, containing a silicon compound andfluoride, is treated to coprecipitate the rare earth from thehomogeneous solution, and the coprecipitate is recovered in the samemanner as described above. The recovered coprecipitate is then convertedto rare earth oxides containing silicon and fluorine by firing thecoprecipitate at a temperature of 550°C. to 1350°C.

A mixture comprising the resulting silicon and fluorine-containing rareearth oxides and a sulfidizing agent is then heated within a temperaturerange of 650°C. to 1350°C. to convert the oxide to a rare earthoxysulfide containing 10 to 1000 p.p.m. silicon and 20 to 500 p.p.m.fluorine in the crystal lattice. We have found that by incorporating thefluorine in this manner rather than by adding a fluoride to the alreadyprepared rare earth oxides, the particle size of the crystals may beheld to about six microns while still maintaining very high luminousefficiency.

Incorporating silicon, and optionally fluorine, in the matrix of rareearth phosphors as above described improves the luminescent efficiencyof the resultant phosphors.

The oxysulfide phosphors according to the invention and as herein abovedescribed advantageously conform in composition, it is believed, to theformula M₂ _(-x) O₂ S:xA in which M is the matrix (host) rare earthelement, A is the rare earth activator, and x is 0.001 to 0.1, thephosphor being modified by incorporation into the crystal latticestructure of 10 to 1000 p.p.m. of silicon and, if used, 20 to 500 p.p.m.of fluorine. Particularly advantageous phosphors of this character arethose in which M is yttrium or gadolinium, and A is europium or terbium.

The phosphors according to the present invention may be used asphotoluminescent, cathodoluminescent and X-ray luminescent phosphors.Particular phosphors may be especially suitable for particular uses.Thus, for example, a cathode ray tube including a red-luminescentviewing screen comprising a substrate may be coated on a surface thereofwith a plurality of rare earth oxysulfide particles of a matrix ofyttrium oxysulfide or gadolinium oxysulfide having as an activatoreuropium and incorporating in the matrix structure from 10 to 1000p.p.m. silicon and, if used, 20 to 500 p.p.m. of retained fluorine. AnX-ray intensifying screen may comprise a substrate coated on a surfacethereof with a plurality of rare earth oxysulfide particles composed ofa matrix of yttrium oxysulfide or gadolinium oxysulfide having as anactivator terbium, advantageously also including 10 to 100 p.p.m. (basedon phosphor weight) of dysprosium and incorporating from 10 to 1000p.p.m. silicon and if used, 20 to 500 p.p.m. of retained fluorine in thematrix structure.

The rare earth oxysulfide phosphors in accordance with this inventionhave, as previously stated, retained silicon and, if used, fluorinewhich are incorporated within the lattice structure of the phosphor. Theprecise manner in which these atoms are associated with and bonded tothe rare earth oxysulfide matrix is not clear. However, we have found byanalysis that these atoms are in fact incorporated in the latticestructure of the phosphor crystal and are not present merely in the formof simple residues of the added silicon compound and fluoride compoundadded. The presence of these atoms in the rare earth oxysulfide crystalstructure is important to achieve the improved phosphor brightness andcontrolled crystal growth which characterize the new product. Theliterature contains numerous references to rare earth oxygen-fluoridecompounds and rare earth sulfur-fluoride compounds, but so far as we areaware no rare earth oxygen-sulfur-silicon-fluorine phosphor structureshave heretofore been known.

The invention is further described in detail in the following examplesfor illustrative purposes:

EXAMPLE 1

Four mixtures of 550 grams La₂ O₃ containing 0.2% Tb₂ O₃ and 10 p.p.m.Dy₂ O₃, 110 grams of Na₂ CO₃, 110 grams of sulfur and 63.3 grams of K₃PO₄ were prepared. Three of the mixtures contained additional reagentsas described below:

a. To one mixture was added 5 × 10.sup.⁻⁴ mole SiO₂ per mole of rareearth oxide. The silica was added in the form of commercially availablecolloidal silica.

b. A second mixture was made to contain 1 × 10.sup.⁻³ mole SiO₂ per moleof rare earth oxide.

c. The third mixture contained 1 × 10.sup.⁻³ mole SiO₂ and 1.1 ×10.sup.⁻² gram atom F per mole of oxide, added as NH₄ HF₂

d. The fourth mixture did not contain any added fluorine or siliconcompounds.

All four of the mixtures were fired in covered alumina crucibles for41/2 hours at 2000°F., and the resultant phosphors were excited andtested for luminescence efficiency (brightness).

The following table gives the results of this experiment: Amount of SiLuminescenceor F Compound AddedEfficiency______________________________________a) 0.05 mole % SiO₂107b) 0.1 mole % SiO₂ 108c) 0.1 mole % SiO₂ + 1.1 mole %F 108d) None100______________________________________

EXAMPLE 2

Three mixtures of the following chemicals were prepared in a blender:550 grams Gd₂ O₃ containing 0.2% Tb₂ O₃ and 10 p.p.m. Dy₂ O₃, 110 gramsNa₂ CO₃, 110 grams sulfur and 63.3 grams of K₃ PO₄.m₂ O. Each of themixtures was treated with additional reagents as described below:

a. One mixture was made to contain 5 × 10.sup.⁻⁴ mole Si per mole ofrare earth oxide. The silicon was added in the form of the compound Na₂SiF₆.

b. To the second mixture was added 5 × 10.sup.⁻⁴ mole Si per mole ofrare earth oxide. The silicon in this case was in the form of SiO₂.

c. The third mixture contained 5 × 10.sup.⁻⁴ mole Si and 3.2 × 10.sup.⁻²mole F per mole of rare earth oxide. The additives were added as SiO₂and NH₄ HF₂.

All three preparations were fired in covered alumina crucibles for 41/2hours at 2100°F. and then tested for brightness as in Example 1.

The following table gives the results of this experiment:

                         Luminescence                                             Si or F Compound Added                                                                             Efficiency                                               ______________________________________                                        a) Na.sub.2 SiF.sub.6                                                                              106                                                      b) SiO.sub.2         100                                                      c) SiO.sub.2 + NH.sub.4 HF.sub.2                                                                   106                                                      ______________________________________                                    

The low luminescence efficiency in this Example of the product producedby adding SiO₂ to the rare earth mixture, as compared with the similarproduct of Example 1, was the result of using gadolinium oxide inExample 2 instead of lanthanum oxide as in Example 1. In the case oflanthanum oxysulfide phosphors, silicon can be incorporated withrelative ease. With gadolinum and yttrium oxysulfide phosphors,incorporation of silicon is difficult in the absence of fluorine.

EXAMPLE 3

A mixed solution containing 4950 milliliters of 0.6-molar GdCl₃, 63milliliters of 1-molar TbCl₃, 2.4 milliliters DyCl₃ solution(corresponding in concentration to 5 grams Dy₂ O₃ /liter) and 34milliliters of NH₄ F.HF solution contaning 0.041 grams F/ml was preparedin a glass container and stirred for 16 hours. The temperature of thesolution was brought to 30°C. and 7750 milliliters of 10% oxalic acidwas added in three equal portions, waiting 5 minutes between eachaddition. The resulting precipitate was then digested while stirring for30 minutes. In the course of this treatment some amount between 10 and1000 p.p.m. of silicon was leached from the walls of the glass containerand incorporated in the precipitate.

The resulting coprecipitate was filtered and washed with deionized H₂ O.The washed coprecipitate was dried for 16 hours at 550°F. and thereafterwas fired for 31/2 hours at 2150°F. to form a silicon and fluorinecontaining oxide.

To this oxide was added Na₂ CO₃, sulfur and K₃ PO₄ and the mixture wasconverted to a terbium-dysprosium activated silicon andfluorine-containing gadolinium oxysulfide phosphor by first mixing,milling, and mixing the mixture and then firing the mixture in 250 cc.alumina crucibles covered with porcelain lids and covered with largesilica crucibles for 41/2 hours at 2100°F.

EXAMPLE 4

A mixed solution containing 10,080 milliliters of 0.6-molar GdCl₃solution. 12.6 milliliters of TbCl₃ solution (conforming inconcentration to 173 grams TB₂ O₃ /liter), and 4.8 milliliters of DyCl₃solution (conforming in concentration to 5 grams Dy₂ O₃ /liter) wasprepared and diluted to 10,100 milliliters. The resulting solution wasdivided into four equal parts of 2525 milliliters each in glasscontainers and treated as follows:

a. One 2525 milliliter portion was brought to a temperature of 30°C. andwhile being stirred a total of 3875 milliliters of 10% oxalic acid wasadded in three equal portions at 5 minute intervals. The resultingcoprecipitate was digested for 30 minutes, filtered and washed withdeionized H₂ O. The coprecipitate was dried at 550°F. and fired at2150°F. for 31/2 hours. Thereafter 100 grams of the fired oxide phosphorwas mixed and milled with 20 grams of Na₂ CO₃. 20 grams sulfur and 11.5grams of K₃ PO₄. The resulting mixture was converted toterbium-dysprosium activated gadolinium oxysulfides as described inExample 1.

b. To a second 2525 milliliter portion was added 34 milliliters of a NH₄F.HF solution containing 0.041 grams F/ml. The solution was digested for16 hours with stirring. Thereafter, the solution was precipitated anddigested as in part (a) above. In the course of this treatment someamount between 10 and 1000 p.p.m. of silicon was leached from the glasscontainer and incorporated in the precipitate. The precipitate wascalcined to oxide and converted to a silicon- and fluorine-containingoxysulfide in the same manner as described in part (a) hereinabove.

c. To a third 2525 milliliter portion was added 68 milliliters of theNH₄ F.HF solution and the resulting solution was precipitated, calcinedand converted to a silicon- and fluorine-containing oxysulfide asdescribed in part (b).

d. To the last 2525 milliliters of the solution described in part (a)was added 102 milliliters of the NH₄ F.HF solution, and the resultingsolution was precipitated, calcined and converted to a silicon- andfluorine-containing oxysulfide as described in part (b).

The silicon- and fluoride-containing oxysulfide phosphors produced inaccordance with parts (b), (c), and (d), when coated on a cathode raytube screen and excited by an electron beam of the same energy in eachcase were all notably brighter than the silicon- and fluorine-freeoxysulfide phosphor made in accordance with part (a) and correspondinglyexcited. Furthermore, as the concentration of fluorine increased in thephosphor of parts (b), (c), and (d), respectively, so did the particlesize of the phosphors.

EXAMPLE 5

The mixed solution containing 8080 milliliters of 0.6 molar YCl₃, 9.65milliliters of 1-molar TbCl₃ and 54.6 milliliters of NH₄ F.HF containing0.041 grams F/ml. or 0.41%F. was prepared and stirred for 16 hours in aglass container.

Thereafter the temperature of one-half of the solution was brought to30°C. and 5450 milliliters of 10% oxalic acid was added in three equalportions, waiting 5 minutes between each addition. The resultingcoprecipitate contained fluorine and some amount from 10 to 1000 p.p.m.of silicon leached from the glass container wall by the fluoride. It wasstirred for 30 minutes, filtered and washed with deionized water. Thewash coprecipitate was dried at 220°F. and thereafter was fired for 31/2hours at 2150°F.

To the remaining one-half of the mixed solution was added an additional27.3 milliliters of NH₄ F.HF containing 0.041 grams F/ml. for a totalfluoride ion concentration of 0.82%. The resulting solution was thencoprecipitated and the coprecipitates fired in the same manner asdescribed in the preceding paragraph.

To 270 gram samples of each of the above-prepared silicon- andfluorine-containing oxide phosphors were separately added 86.7 gramssodium carbonate, 86.7 grams sulfur and 50 grams K₃ PO₄. These twobatches of ingredients were then fired to form silicon- andfluorine-containing oxysulfide phosphors in the same manner as describedin Example 4.

The terbium-activated yttrium oxysulfide phosphors made with 0.41%F.added had a particle size of 6.34 microns whereas the phosphors madewith 0.82%F. added had a particle size of 6.8 microns. Both of thesesilicon- and fluorine-containing phosphors showed high brightness ofblue-white color when excited by electrons in a cathode ray tube.

EXAMPLE 6

A mixed solution containing 5620 milliliters of 0.6 molar LaCl₃, 6.75milliliters 1-molar TbCl₃, 2.4 milliliters DyCl.sub. 3 of concentrationequivalent to 5 grams Dy₂ O₃ /liter, and 38.6 milliliters NH₄ F.HFcontaining 0.041 g/ml of F was prepared and stirred for 16 hours in aglass container.

Thereafter, the temperature of the solution was brought to 50°F. and7750 milliliters of 10% oxalic acid was added. The resultingcoprecipitates, containing occluded silicon (from the glass containerwall) and fluoride as discussed in the previous Examples, were stirredfor 30 minutes, filtered and washed with deionized water. Thereafter,the coprecipitates were dried at 550°F. and fired for 3 hours at 2150°F.to form a silicon- and fluoride-containing oxide phosphor.

To 278.5 grams of the oxide phosphor was added 55.2 grams Na₂ CO₃, 55.2grams sulfur and 19.5 grams K₃ PO₄. These ingredients were mixed, milledand mixed again. Thereafter, the ingredients were placed in coveredalumina crucibles and fired for 3 hours at 2100°F.

The terbium-dysprosium activated lanthanum oxysulfide phosphors producedby this method contained somewhere between 10 and 1000 p.p.m. siliconand about 100 p.p.m. fluorine, had a particle size of 9.89 microns andexhibited bright yellow-green luminescence on cathode ray excitation.

EXAMPLE 7

A mixed solution containing 5620 milliliters 0.6-molar LaCl₃, 6.75milliliters 1-molar TbCl₃ (0.2 mole %), 2.4 milliliters Dy₂ O₃containing 5 grams Dy₂ O₃ (22 p.p.m.) and 77.2 milliliters NH₄ F.HFcontaining 0.041 g/ml of F (about 0.6%) was prepared and stirred for 16hours in a glass container.

Thereafter, the temperature of the solution was controlled to 50°F. and7750 milliliters of 10% oxalic acid were added. The resultingcoprecipitates were stirred for 30 minutes, filtered and washed withdeionized water. Thereafter, the coprecipitates, containing siliconleached from the glass container wall and fluorine, were dried at 550°F.and then fired for 3 hours at 2150°F. to form silicon- andfluoride-containing oxide phosphor. To 276 grams of this oxide phosphorwas added 54.6 grams Na₂ CO₃ 54.6 grams sulfur and 19.3 grams K₃ PO₄.These ingredients were mixed, milled and mixed again. Thereafter theingredients were placed in covered alumina crucibles and fired for 3hours at 2100°F.

The terbium-dysprosium activated lanthanum oxysulfide phosphors producedby this method contained somewhere between 10 and 1000 p.p.m. siliconand about 100 p.p.m. of fluorine in the crystal lattice, had a particlesize of 10.5 microns, and exhibited bright yellow-green luminescence oncathode ray excitation.

EXAMPLE 8

Two separate mixed solutions each containing 10,080 milliliters of0.6-molar GdCl₃, 12 milliliters of 1-molar TbCl₃, 4.8 milliliters DyCl₃solution containing the equivalent of 5g/l Dy₂ O₃, and 136 millilitersNH₄ F.HF containing 0.041 g/ml (0.5%F) were prepared and stirred for 16hours in a glass container. The temperature of the solutions wasadjusted to 30°C. and 7750 milliliters of 10% oxalic acid was added inthree equal parts with a five minute interval between each addition. Theresulting coprecipitate containing silicon and fluorine was stirred for30 minutes, filtered and then washed wih deionized water. Thereafter,the coprecipitate was dried at 550°F. and then fired for 31/2 hours at2150°F.

a. To 350 grams of the resulting oxide was added 50 grams Na₂ CO₃, 50grams sulfur, 28.8 grams K₃ PO₄, 7.17 grams NaF and 2.7 grams NH₄ Cl,and the ingredients were mixed.

b. The same mixture was prepared as in part (a) except that only 3.85grams NaF and 1.38 grams NH₄ Cl were used.

c. The same mixture was prepared as in part (a) except that 2.5 grams ofNH₄ F.HF was used instead of NaF and NH₄ Cl.

The composition of parts (a), (b), and (c) were treated individually bymixing, milling, and mixing the ingredients and thereafter by beingfired in covered alumina crucibles and 41/2 hours at 2100°F.

All three terbium-dysprosium activated gadolinium oxysulfide phosphorscontaining silicon and fluorine prepared above exhibited paleyellow-green emission when excited by electrons, X-rays, and 2537Aultraviolet. The particle size and fluoride concentration in the crystallattice of the samples produced in parts (a), (b), and (c) was found tobe as shown in the following table:

                              Fluoride                                                      Particle Size   Concentrations                                      Phosphor  (Microns)       (p.p.m.)                                            ______________________________________                                        a)        17.6            65                                                  b)        13.9            25                                                  c)        12.97           20                                                  ______________________________________                                    

The above table illustrates that the fluoride concentrations foundcorrelate with the amount added. There is also an increase in particlesize with increasing fluoride concentration.

While the fluoride addition is not in all cases essential, it ispreferred in order to facilitate forming the silicon-containingoxysulfide phosphors of the rare earth metals gadolinium and yttrium.

While it is not precisely understood how the silicon and (if used)fluorine substitute in the phosphor matrix, they are present therein andtheir presence demonstrably effects a large increase in the luminescenceefficiency (brightness) of the phosphors.

While the invention has been described in connection with a preferredembodiment, it is not intended to limit the invention to the particularform set forth, but, on the contrary, it is intended to cover suchalternatives, modifications and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claims.

What is claimed is:
 1. A cathode ray tube including a luminescent viewing screen comprising a substrate having on a surface thereof a plurality of particles of a rare earth oxysulfide phosphor consisting essentially of a rare earth oxysulfide matrix of the formula M₂ _(-x) O₂ S:xA wherein M is at least one rare earth element selected from the group consisting of lanthanum, gadolinium and yttrium, A is at least one rare earth activator selected from the group consisting of praseodymium, neodymium, samarium, europium, terbium, dysprosium, holmium, erbium, ytterbium, and thulium, and x is 0.001 to 0.1, silicon incorporated in said matrix in an amount from about 10 to 1000 p.p.m. based on phosphor weight and, when M is gadolinium or yttrium, fluorine incorporated in said matrix in an amount from about 20 to 500 p.p.m.
 2. A cathode ray tube according to claim 1 wherein the phosphor particles are crystalline compounds having the formula

    Y.sub.(2.sub.-x) O.sub.2 S:xEU.


3. a cathode ray tube according to claim 1 wherein the phosphor particles are crystalline compounds having the formula

    Gd.sub.(2.sub.-x) O.sub.2 S:xEu.


4. A cathode ray tube according to claim 1 wherein the phosphor particles are crystalline compounds having the formula

    Y.sub.(2.sub.-x) O.sub.2 S:xTb.


5. A cathode ray tube according to claim 1 wherein the phosphor particles are crystalline compounds having the formula

    Gd.sub.(2.sub.-x) O.sub.2 S:xTb. 